The mean free path and collisions
At room temperature, the rms speed of O2 or N2 is about 500 ms-1. Processes such as the diffusion of one gas into another would therefore be almost instantaneous, were it not for the occurrence of collisions between molecules. Collisions are fundamentally quantum mechanical events, but in a dilute gas, molecules spend most of their time between collisions and so we can consider them as classical billiard balls and ignore the details of what actually happens during a collision. This chapter models the effect of collisions in a gas and develops the concepts of a mean collision time, the collision cross-section and the mean free path.
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